Hitherto, there have been mainly produced two types of golf balls. The one is a solid golf ball, such as a two-piece golf ball, which is composed of a core formed from integrally molded rubber material and a thermoplastic resin cover (e.g. ionomer resin cover) formed on the core. The other is a thread wound golf ball which is composed of a solid or liquid center, a thread rubber wound layer formed on the center and a cover of ionomer resin or balata etc. having a thickness of 1 to 2 mm covering on the thread rubber wound layer. The solid golf ball, when compared with the thread wound golf ball, has better durability and better flight performance because of larger initial velocity at the time of hitting and longer flight distance. Therefore, the solid golf ball is generally approved or employed by many golfers, mainly amateur golfers. On the other hand, the two-piece solid golf ball exhibits a hard shot feel at the time of hitting.
In order to improve the shot feel of the two-piece solid golf ball, it has been attempted to soften the cover or the core of the solid golf ball. However, the softening of the cover or core adversely sacrifices flight distance inherent in the two-piece solid golf ball. It has also been proposed that the core or cover is made of a plurality layered to improve shot feel. However, there is the drawback that production efficiency is poor, because the method of making the plural layered core or cover is complicated.
The present inventors have proposed a solid golf ball having a hollow at its center (Japanese Patent Kokai Publication No. 308709/1997). The hollow solid golf ball has a hollow portion having a diameter of 5 to 30 mm and the hollow portion is surrounded by rubber which is formed from a rubber composition which has been conventionally used for cores of solid golf balls. Because of the presence of the hollow portion, the weight of the golf ball is reduced. This reduction in the ball weight is compensated for by the addition of a high specific gravity metal powder to the rubber composition.
In the hollow golf ball, impact absorption is improved by making the core hollow, and thus shot feel is improved. The hollow golf ball also has excellent flight performance and maintains a long flight distance because the weight is distributed around the outer portion of the golf ball and the moment of inertia of the golf ball is increased, thus increasing the launch angle and reducing the spin amount immediately after hitting, but reducing spin attenuation. The hollow golf ball has excellent physical properties as described above, but the present inventors further try to improve the properties even more.
For example, impact absorption, which is one of important technical effects of the hollow golf ball, can be improved by enlarging the hollow portion. However, the enlargement of the hollow portion degrades the rebound characteristics and reduces flight distance. Also, it increases deformation of the cover or core outer layer at the time of hitting, thus reducing ball durability and degrading cut resistance.